src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/gc/z/ZPage.java

*** 33,42 ****
--- 33,44 ----
  import sun.jvm.hotspot.types.TypeDataBase;
  
  public class ZPage extends VMObject {
      private static CIntegerField typeField;
      private static long virtualFieldOffset;
+     private static CIntegerField topField;
+     private static CIntegerField refcountField;
      private static long forwardingFieldOffset;
  
      static {
          VM.registerVMInitializedObserver((o, d) -> initialize(VM.getVM().getTypeDataBase()));
      }
*** 44,53 ****
--- 46,57 ----
      static private synchronized void initialize(TypeDataBase db) {
          Type type = db.lookupType("ZPage");
  
          typeField = type.getCIntegerField("_type");
          virtualFieldOffset = type.getField("_virtual").getOffset();
+         topField = type.getCIntegerField("_top");
+         refcountField = type.getCIntegerField("_refcount");
          forwardingFieldOffset = type.getField("_forwarding").getOffset();
      }
  
      public ZPage(Address addr) {
          super(addr);
*** 63,76 ****
  
      private ZForwardingTable forwarding() {
          return (ZForwardingTable)VMObjectFactory.newObject(ZForwardingTable.class, addr.addOffsetTo(forwardingFieldOffset));
      }
  
!     private long start() {
          return virtual().start();
      }
  
      Address forward_object(Address from) {
          // Lookup address in forwarding table
          long from_offset = ZAddress.offset(from);
          long from_index = (from_offset - start()) >> object_alignment_shift();
          ZForwardingTableEntry entry = forwarding().find(from_index);
--- 67,88 ----
  
      private ZForwardingTable forwarding() {
          return (ZForwardingTable)VMObjectFactory.newObject(ZForwardingTable.class, addr.addOffsetTo(forwardingFieldOffset));
      }
  
!     public long start() {
          return virtual().start();
      }
  
+     public long top() {
+         return topField.getJLong(addr);
+     }
+ 
+     public int refcount() {
+         return topField.getJInt(addr);
+     }
+ 
      Address forward_object(Address from) {
          // Lookup address in forwarding table
          long from_offset = ZAddress.offset(from);
          long from_index = (from_offset - start()) >> object_alignment_shift();
          ZForwardingTableEntry entry = forwarding().find(from_index);